1. Field of the Invention
The present invention relates to cathodic protection measurement apparatus for measuring the potential difference between a buried structure receiving cathodic protection and the surrounding soil.
2. Description of the Related Art
Corrosion of a buried metal structure is a destructive attack on the metal, which is generally electrochemical in nature. The mechanism of electrochemical corrosion occurs in two parts: (1) an anodic reaction, in which the metal dissolves in an electrolyte in the form of positively charged ions, and (2) a cathodic reaction, in which positively charged hydrogen ions plate out as atomic hydrogen on the cathodic surface. The electrons released by the anodic reaction flow through the metallic circuit to the cathode, where they neutralize an exactly equivalent number of hydrogen ions. Thousands of microscopic corrosion cells are formed along the surface of the metal structure, where the metal structure serves as an anode and the soil serves as an electrolyte. Thus, the metal dissolves by releasing electrons into the soil to neutralize hydrogen ions.
The metal may be coated with zinc, tin, lead, nickel, copper, chromium, aluminum or various other types of coatings applied using a variety of processes, including galvanizing, sherardizing, metal spraying or metalizing, electrolytic, cold process or hot processes as known to those skilled in the art. An appropriate coating can retard or almost eliminate corrosion over most of the surface of the buried structure. However, small imperfections or scratches in the coating expose the bare metal to the soil. The exposed locations and other discontinuities of the coating are generally referred to as "holidays," which become anodic and begin the corroding process.
Cathodic protection is a technique used to protect buried metal structures from corrosion. A cathodic protection system protects a structure by applying a DC voltage, causing the structure to become cathodic by collecting current. The DC voltage forces current to flow from a purposely established ground connection through the earth and onto the structure to be protected. There are two primary types of cathodic protection systems. The first type uses sacrificial or galvanic anodes, which are made of a metal that is more negative in the galvanic series than the metal to be protected. The galvanic series is a measure of the tendency of a metal to dissolve in water, known as solution pressure, which can be measured by the amount of electrical potential that must be applied to prevent dissolution when the metal is dissolved in one of its salts at standard concentration. When two metals are immersed in an electrolyte, such as soil, that metal which is above the other in the series becomes anodic, suffers corrosion, and protects the other metal by rendering it cathodic.
A second type of cathodic protection is an impressed current system. An impressed current system uses an outside source of DC voltage and current, which is connected with its negative terminal to the buried metal structure to be protected, such as a buried pipeline, and with its positive terminal connected to one or more earth contacting anodes or ground beds made of a material having a low rate of dissolution. Current is forced to flow from the ground bed and through the electrolyte to the metal pipeline. Applied voltage and circuit resistances may be adjusted to permit circulation of the required amount of current to attain full protection. Impressed current systems are implemented in several different ways depending upon the available source of power. If commercial AC power source is available, a rectifier is used to convert the AC power to a low voltage DC source. Other means of electric power may be used, such as engines, turbines, or thermal generators, fuel cells, or even batteries when other sources of power are not practical.
Once a cathodic protection system is in place, the level of cathodic protection must be periodically tested to assure proper protection. The potential difference between a buried metal pipeline and the soil is of considerable importance, either in investigating the corrosive conditions or in evaluating the extent of cathodic protection being applied. The potential difference, often referred to as the pipe-to-soil (P/S) potential, is measured while cathodic protection is being applied. The P/S potential is measured by connecting a measuring instrument between the pipeline through direct metal contact and a reference electrode placed in contact with the soil. A series of test points or test stations are provided along the pipeline, where each test point includes a test wire electrically coupled to the pipeline and brought to the surface through a hollow access tube, usually made of plastic or poly-vinyl-chloride (PVC). The electrical connection of the test wire to the pipeline is preferably made by a welded or soldered lead. The reference electrode used for measuring the potential difference is a half-cell having a known voltage level for establishing a voltage reference, where the half-cell is preferably a copper-copper sulfate half-cell known to those skilled in the art.
To measure the level of cathodic protection, the reference electrode is preferably placed on the soil as close to the metal pipeline as possible. It is desired to penetrate the soil down to within about 6 inches of the pipeline, but this is often not convenient or feasible. Usually, the reference electrode is placed directly above the buried metal pipeline on the surface of the ground. In general, the test wire is connected to the negative terminal of a high resistance voltmeter, having its positive terminal connected to the terminal of the reference electrode placed on the ground. In this manner, a galvanic cell or small battery is achieved through connecting two half-cells together, the first half-cell being the natural half cells formed between the pipeline and the soil, and the second half-cell being the reference electrode.
There are several errors that are introduced when taking cathodic protection measurements as described above. The errors are primarily caused by I/R voltage drops while the cathodic protection system is activated and current is flowing. An I/R voltage drop is simply a voltage drop caused by current flow through a resistive element in accordance with Ohm's law. Resistive elements include test lead connections, the soil between the pipeline and the reference electrode, the interface between the pipeline and its coating and other physical elements. Proper practice can reduce I/R voltage drop caused by the test leads to some extent. The I/R voltage drop due to the soil electrolyte can be substantially eliminated by digging a hole and placing the reference electrode within six inches of the pipeline. As described above, however, this is often not practical. Even if all appropriate procedures are practiced, I/R voltage drop may still be substantial if the cathodic protection current is significant. The total I/R voltage drop must be estimated or measured and subtracted out or otherwise accounted for.
The only known way to completely eliminate the I/R voltage drops to is deactivate the cathodic protection system to stop current flow. This, however, also removes the cathodic protection being measured. Nonetheless, if all cathodic protection currents are interrupted simultaneously while monitoring the voltage potential, an initial instantaneous voltage drop is observed, which is approximately equal to the total I/R voltage drop. The new instantaneous voltage level is the true P/S potential indicating the level of cathodic protection, plus the voltage of the reference electrode. This "I/R-free" P/S potential does not last, however, because another voltage shift begins to occur due to depolarization.
Polarization is a desirable effect that occurs at the P/S junction, where ions are reduced to hydrogen as a result of the passage of current directly to or from an electrolyte. Polarization often takes hours or even days to stabilize after cathodic protection is applied. After the cathodic protection potential is removed, depolarization begins causing a decay or decrease in the measured P/S potential. The depolarization occurs more quickly at first, then gradually decreases and stabilizes over time. A minimum negative polarization voltage shift of 100 millivolts (mV) is often specified to indicate adequate cathodic protection. Thus, it is desirable to measure both the I/R free P/S potential and the amount of depolarization.
Temporarily stopping the current flow is difficult, if not impossible, in a galvanic anode system. The conductor between the metal pipeline and each galvanic anode must be readily accessible, and each one of the anodes must be accessed and terminated simultaneously while taking measurements. Often, however, the anodes are buried along with the conductor between each anode and the metal pipeline, so that the conductors are inaccessible. Even if accessible, it would be substantially difficult to locate and simultaneously turn off each anode while taking test measurements. This is usually not practiced.
Temporarily and simultaneously terminating all cathodic current flow in an impressed current system is somewhat easier than for a galvanic anode system. Usually, the power sources are located above ground and are sometimes even designed to be switched off for the purpose of cathodic protection measurements. Of course, all of the power sources must be switched off simultaneously to achieve the most accurate measurements. Impressed current systems are presently in use which use a current interruption procedure, where a particular measurement schedule is defined. During the measurement period, all sources of cathodic protection are synchronously switched on and off periodically. The basis of this interruption method is that I/R voltage drops immediately become zero, permitting I/R free readings before significant depolarization begins. Various duty cycles are possible to achieve fast or slow cycle interruption. For example, a three hour period may be scheduled on a particular day, where each of the power sources remain on for 45 seconds and then are turned off for 15 seconds for each minute during the entire three hour measurement period. This is an example of a slow cycle. In a fast cycle interruption procedure, the rectifiers are shut off for a much shorter period of time, such as 0.1 seconds of each 0.6 seconds. Many various schemes are possible. Thus, a technician aware of the interruption schedule can take the appropriate measurements at the appropriate time.
The current interruption techniques for impressed current systems are rather elaborate and very expensive. Such a system is only feasible if designed initially, and then only if the expense can be justified. Many pipelines and cathodic protection systems exist which do not use these techniques. Also many projects cannot afford them. Further, current interruption schemes are not always convenient, since the technician must be aware of and be available during the scheduled measurement periods. Sophisticated electrical equipment is often required to take the measurements, especially for fast interruption cycles. Therefore, current interruption techniques used in impressed current systems have relatively limited use.
It is desired to provide a means for measuring cathodic protection without I/R voltage drop errors for all types of cathodic protection systems, by accurately measuring the potential difference between a buried structure and the surrounding soil. Further, it is desired to make such measurements conveniently and without excessive cost.